[rab9]

Thanks so much everyone for this specific information!

I am using benzoic acid at the moment. I’m aware it does have poor solubility but I use a mini milk frother to blend it into my hot water phase and it does the job. I’ve been using it a long time, have been resistant to change (even more resistant after a very unsuccessful round of experimenting with several ‘natural’ alternatives) but I have recently purchased some sodium benzoate. I have not tested it yet. (I understand that sodium benzoate releases benzoic acid anyway when combined with water so my concern about a reaction with MgCl would still be relevant when I make the changeover)

Firstly, thanks for the suggestions on preservatives that are specifically benign to MgCl2, that is extremely helpful for this product that I wish to make. 
I also appreciate everyone being so emphatic about the need for strong preservation, and taking your duty of care seriously by pointing out the weakness in my preservative of choice. I’m aware of benzoic acid’s weakness against bacteria and finding an effective combination of preservatives that also align with my values (ethical/sustainable, preferably no parabens or formaldehyde) is a priority for me but it’s been a bit of a long process for me to sort through the minefield that is the trend towards ‘natural’ ingredients, the propaganda that has accompanied that and my own bias. 
@MarkBroussard : I was happy to google EHG and discover it has no parabens or formaldehyde. Thank you for the specific suggestion.

This forum has been a valuable source of rational information regarding preservatives, being offered by people that don’t have an agenda other than to make safe products. Thank you for that.

It seems that if I decide to use Phenoxyethanol/EHG I would still need an anti fungal and I know that my ‘old faithful’ Benzoic acid (or sodium benzoate when I make the change) is a great antifungal, perhaps they would support each other well.  I will do my own research but if anyone feels it appropriate to comment on that combination I’d appreciate it.

I have gone off track from my original query, (please forgive) but it was somewhat relevant to suggestions that were made.

My main concern was that I might ‘lose’ my MgCl2 or the preservative properties of the benzoic acid to a chemical reaction if used in combination, or just end up with a substance unpleasant for topical use 
@ketchito and @PhilGeis “Mg2+ counterion would interact with the carboxylic part of the acid. Nevertheless, at enough low pH, you’ll have the free acid back.”
Does this mean I don’t need to be concerned about this if my pH is low enough and that I could use sodium benzoate and MgCl2 in combination without worrying about ‘loss’ through chemical reaction?
Would you consider pH of 4 - 4.5 low enough? Thanks
Perhaps I would be better off researching an alternative anti-fungal to support phenoxy/EHG that is also benign to MgCl2…… (not an organic acid)

The heads up regarding how MgCl2 affects viscosity is helpful, thank you. I will keep that in my awareness and make small batches to experiment, perhaps keeping it lower than 30%.

Also very helpful to know that my MgCl2 may be ‘trapped’ in the water phase of my emulsion anyway.  I am going to do a bit more research on this.

I am very grateful for your time everybody, you have given me some fantastic additional relevant information that will help me in my research and experimentation of this product.